Astron. Astrophys. 344, 43-50 (1999)

Astron. Astrophys. 344, 43-50 (1999)

Properties of the host galaxy of the gamma-ray burst 970508 and local star-forming galaxies

V.V. Sokolov¹, S.V. Zharikov¹, Yu.V. Baryshev², M.O. Hanski³, K. Nilsson³, P. Teerikorpi³, L. Nicastro⁴ and E. Palazzi⁵

¹ Special Astrophysical Observatory of R.A.S., Karachai-Cherkessia, Nizhnij Arkhyz 357147, Russia (sokolov,zhar@sao.ru)
² Astronomical Institute of St.Petersburg University, St.Petersburg 198904, Russia (yuba@aispbu.spb.su)
³ Tuorla Observatory, University of Turku, FIN-21500 Piikkiö, Finland (mihanski,kani,pekkatee@astro.utu.fi)
⁴ Istituto di Fisica Cosmica con Applicazioni all'Informatica, CNR, Via U. La Malfa 153, I-90146 Palermo, Italy
⁵ Istituto Tecnologie e Studio Radiazione Extraterrestre, CNR, Via Gobetti 101, I-40129 Bologna, Italy

Received 14 September 1998 / Accepted 18 December 1998

Abstract

Recent observations of GRB970508 with the SAO-RAS 6-m telescope in July-August 1998 show that the optical transient (OT) has varied very little since November 1997. Here we report on the behaviour of the [FORMULA] light-curves for up to 470 days from the date of the GRB occurrence. After days any power-law decay has ceased and the OT contamination of the host galaxy flux is already less than the observational errors. We derive the broad band spectrum of the host galaxy, without OT, and compare it with the average continuum spectra of galaxies of different Hubble types. The spectral distribution of normal galaxies with types earlier than Sbc is confidently excluded. For [FORMULA] km s^-1 Mpc^-1 and three Friedmann models with matter density and cosmological constant parameters (1,0); (0,0); (0,1) the derived host galaxy absolute magnitudes () are , and respectively. The absolute B magnitude corresponds to the luminosity where is the knee of the local general Schechter luminosity function. However, the luminosity of the GRB host is roughly at the knee of the local luminosity function [FORMULA] for late type galaxies Sd-Sm-Irr (Binggeli et al. 1988). Comparison of the spectral energy distribution of the GRB host with local starburst galaxies leads to best fits for the Scd starburst H II galaxy NGC7793 and the blue compact galaxy Mrk 1267. The position of the host galaxy in the vs. [FORMULA] diagram for local late-type LEDA (Lyon-Meudon Extragalactic database) galaxies allows us to attribute the GRB970508 host galaxy to the blue compact galaxies.